In the past several years researchers noticed that Earth's crust contains too great a ratio of neodymium 142 compared to chondrites.

Seeking to show that the Earth isn't an oddball, Caro and his team turned to Mars and reviewed old data from Earth's moon.

"We found that Martian and lunar rocks are also characterized by an excess in neodymium 142 compared with chondrites," he said.

All Shook Up?

Supporters of the idea that the inner planets formed from chondritic materials have long speculated that the Earth's turbulent history could be to blame for its chemical differences.

Earth regularly shakes up its crust and mantle through plate tectonics and convection—which could have buried reservoirs of material that would balance out the elemental ratio, the scientists argue.

Mars and the moon haven't put their surfaces through the same grinders, however, and yet also appear to have excess neodymium 142.

Car and his team say the difference could come from erosion of planetary crusts in the bodies' formative years. Or the inner planets might have formed long before the rocky bodies of the outer solar system.

Not Quite Settled

Vinciane Debaille, of NASA's Lunar and Planetary Institute in Houston, and her colleagues published a paper in the November 22, 2007, issue of Nature that paints a different picture.

They studied the same class of Mars rocks used in the newer study and agreed that they differ from chondrites.

But her team suggests that early Mars had an insulating atmosphere that kept the planet's interior warm, thereby sustaining a molten magma ocean up to 110 million years after the solar system's formation.

This could have created underground magma reserves rich in the "missing" isotopes.

Richard Carlson, from the Carnegie Institution of Washington, says he hasn't given up on the idea that Earth could be harboring chondrite-like deposits close to its core.

For instance, ancient rocks from Greenland differ from rocks elsewhere on Earth and have different ratios of neodymium isotopes, he points out.

And he hesitates to draw any conclusions about all of Mars from a small sample of its rocks.

"To think that we can get definitive information about the bulk composition of Mars from a handful of meteorites," he said, "all likely from the same area of the Martian crust, is very optimistic."